for my gallery I have used a typical button filled with picture an text.
When you mouse over a banner appears indicating name of project
before you click to go on that target project.
at screen 320 I have used text size 8 for the above banner
at screen 1500 I have used text size 14.
When I test from 320 to 1500, text size 8 goes on the banner @ 1500
and when I test from 1500 to 320 text size 14 goes on the banner @ 320.
Furthermore, pictures within the buttons also reflect such behaviour
by expanding and contracting, and loosing their positions.
Does any one knows?
Would you mind to share a .muse with us?
Just one page with one gallery with maybe one, two or three images as you want it to happen in muse?
To do this, simply save your file into your Creative Cloud Files on your machine and by right click share that link with us.
There´s one option for text in the tools panel and the tool "text".
Here you can either check for text formation page wide or breakpoint wide.
Maybe this helps?
Also please do not use a breakpoint at 320. Just use it as the minimum width, as there are no smaller devices than 320 px.